The present invention relates to methods and instruments that determine thin film properties from optical measurements.
Thin solid films are of tremendous industrial importance in a variety of fields, including semiconductor device manufacturing, flat panel display manufacturing, and magnetic head manufacturing. These films commonly consist of mixtures of elements where the concentrations are varied to control the properties of the films (e.g., electrical, mechanical, tribological). Other important parameters used to control the properties of the films include the thickness and optical constants (index of refraction n and coefficient of extinction k) of the films. As a result, when thin films with multiple elemental constituents are used in production applications, it is often crucial to monitor these parameters to achieve the desired properties.
Commercially available spectral reflectometers and ellipsometers are capable of determining thicknesses and optical constant spectra of thin films by measuring reflectance and ellipsometric parameters psi and delta in the spectral ranges of deep ultra-violet (DUV), e.g., approximately 190 to 400 nm (nanometer), visible (VIS), e.g., approximately 300 to 800 nm, and near infrared (NIR), e.g., approximately 800 to 2500 nm. However, spectral reflectometers and ellipsometers in many cases cannot accurately measure elemental concentration. On the other hand, commercially available Fourier Transform Infrared (FTIR) spectrometers are capable of determining elemental concentration of thin films by measuring reflectance or transmittance in the infrared (IR) spectral range, e.g., approximately 2500 to 25,000 nm. Unfortunately, FTIR spectrometers necessitate the prior knowledge of the film layer thicknesses to accurately determine elemental concentration. Accordingly, a user must first use spectral reflectometers and ellipsometers to determine film layer thicknesses and optical constant spectra, and then use FTIR spectrometers to determine elemental concentration.
The use of multiple instruments in many industries, e.g., semiconductor manufacturing, is undesirable because of the high fabrication and maintenance costs of clean rooms. Further, the use of multiple instruments creates time-consuming transfer of samples from one instrument to another and reduces the accuracy of the instruments because it is difficult for multiple instruments to measure from the exact same area on the samples. Thus, there is a need for an integrated optical measuring instrument and methods that determine thickness, optical constant spectra, and elemental concentration of thin films while minimizing space, increasing throughput, and improving accuracy.
An instrument and methods are used to determine film layer thicknesses, optical constant spectra of film layers, and elemental concentrations of a sample substrate overlaid with single or multiple films, in-situ or ex-situ, rapidly, non-destructively, without contact, and on a real-time basis. The instrument measures the sample substrate""s absolute reflectance and ellipsometric parameters psi and delta over a first set of wavelengths, e.g., DUV, VIS, and NIR spectral ranges, to determine film layer thicknesses and optical constants of the film layers over the first set of wavelengths. In one embodiment, absolute reflectance is measured in accordance with the method described in U.S. Pat. No. Re. 34,783, entitled xe2x80x9cMethod for Determining Absolute Reflectance of a Material in the Ultraviolet Range,xe2x80x9d issued to Vincent J. Coates, which is hereby incorporated by reference in its entirety. The instrument also measures the sample substrate""s absolute reflectance or transmittance over a second set of wavelengths, e.g., IR spectral range. Based on these measurements and analysis, the instrument determines one element""s concentration in one or more film layers of the sample substrate.
In one embodiment, the instrument determines one element""s concentration in a film layer of a film layer structure on a sample substrate by correlating the element""s concentration to the film layer""s optical constants in the spectral region of an absorption band of that element. The instrument determines the optical constants using the determined film layer thicknesses to fit an optical model to the measured absolute reflectance or transmittance.
In another embodiment, the instrument determines one element""s concentration in a film layer of the film layer structure on the sample substrate by relating the element""s concentration to the film layer""s thickness and an absorption-related feature of the element from the measured absolute reflectance or transmittance. The instrument determines this relationship by analyzing calibration films with known film layer thicknesses and concentrations of the element.
In yet another embodiment, the instrument determines one element""s concentration in a film layer of the film layer structure on the sample substrate using a trained artificial neural network (ANN) computer program. The embodiment includes an ANN computer program that is trained with optical constants over the first set of wavelengths, absolute reflectance or transmittance over the second set of wavelengths, film layer thicknesses, and elemental concentrations from calibration samples. Thereafter, the instrument determines the element""s concentration in the film layer from the sample substrate""s determined film layer thicknesses, determined optical constants over the first set of wavelengths, and measured absolute reflectance or transmittance over the second set of wavelengths.